1. Field of the Invention
The present invention relates generally to an encoder device, and more particularly, to an optical encoder device that is thinner and at the same time provides highly accurate encoder output.
2. Description of the Related Art
In a magnetic disk drive, for example, a head carriage having a magnetic head is moved in a radial direction of a disk and the magnetic head is made to trace a selected track as the position of the head carriage is detected by the encoder device. Efforts are underway to make encoder devices of this type thinner and more compact while at the same time providing highly accurate encoder output.
The conventional encoder device has two light-receiving elements such as photodiodes placed 90 degrees apart, with two signals phase A and phase B having the same periods being output from light-receiving elements that receive the light from the light-emitting elements. From the two signals phase A and phase B the direction and distance that the head carriage has traveled is obtained.
More recently, in an effort to obtain more accurate encoder device output, four light-receiving elements have come to be used and four signals phase A, phase B, inverted phase A and inverted phase B extracted and the phase A and inverted phase A, as well as the phase and the inverted phase B, are differentially amplified.
FIG. 1 is a schematic structural diagram of a conventional encoder device. As indicated in the diagram, in the conventional encoder device 1 the light-receiving element 2A and the light-receiving element 2B are disposed so as to have phases 90 degrees different from each other, the light-receiving element 2a of the inverted phase A and the light-receiving element 2A are disposed so as to have phases 180 degrees different from each other and the light-receiving element 2b of the inverted phase B and the light-receiving element 2B are disposed so as to have phases 180 degrees different from each other.
Additionally, in the conventional encoder device 1 a light-emitting element 3 is disposed at a location opposite the light-receiving elements 2A, 2B, 2a and 2b, the light-receiving elements 2A, 2B, 2a and 2b symmetrically disposed with respect to a center line of the light-emitting element 3. A main scale 5 made of a single piece of plastic is provided between a lens 3a of the light-emitting element 3 and the light-receiving elements 2A, 2B, 2a and 2b. The main scale 5 has slits 4 spaced at regular intervals, the slits 4 being shown in FIG. 1 as blank openings in the main scale 5.
Light emitted from the light-emitting element 3 is diffused at predetermined angles by the lens 3a so as to reach the light-receiving elements 2A, 2B, 2a and 2b. When the main scale 5, which is movable, moves in a direction D with respect to the light-emitting element 3, the light emitted from the light-emitting element 3 passes through the slits 4 in the main scale 5 and strikes the light-receiving elements 2A, 2B, 2a and 2b. The intensity of the light received at each of the light-receiving elements 2A, 2B, 2a and 2b varies as the main scale 5 moves and its position changes with respect to the light-receiving elements 2A, 2B, 2a and 2b. 
As a result, a waveform signal is obtained from each of the light-receiving elements 2A, 2B, 2a and 2b which corresponds to variations in the level of light received at the light-receiving elements 2A, 2B, 2a and 2b as the main scale 5 changes position with respect to the light-receiving elements 2A, 2B, 2a and 2b. Signals from the light-receiving elements 2A, 2B, 2a and 2b are input into a circuit not shown in the diagram, so that a phase A signal output from the light-receiving element 2A and a phase a signal output from the light-receiving element 2a are differentially amplified to obtain an Axe2x80x2 phase signal (=Axe2x88x92a) and, similarly, a phase B signal output from the light-receiving element 2B and a phase b signal output from the light-receiving element 2b are differentially amplified to obtain a Bxe2x80x2 phase signal (=Bxe2x88x92b). The Axe2x80x2 phase signal and the Bxe2x80x2 phase signal have phases 90 degrees different from each other.
The arrangement of the light-receiving elements 2A, 2B, 2a and 2b is not important so long as phase A signals and phase B signals having phases 90 degrees different from each other and having the same period are output from the encoder device.
FIG. 2 is a diagram showing the conventional arrangement of the light-receiving elements 2A, 2B, 2a and 2b. As shown in the diagram, light-receiving element 2B is positioned to one side of light-receiving element 2A so as to have a phase 90 degrees different from that of light-receiving element 2A, and light-receiving element 2a is positioned to one side of light-receiving element 2b so as to have a phase 90 degrees different from that of light-receiving element 2b. 
By positioning light-receiving elements 2A, 2B, 2a and 2b as described above, the light-receiving elements 2A, 2B, 2a and 2b are spaced an equal distance apart, that is, are spaced so as have a phase difference of 90 degrees. With such an arrangement of the light-receiving elements 2A, 2B, 2a and 2b, interference between the light-receiving elements 2A, 2B, 2a and 2b can be reduced and the sensitivity of the light-receiving elements 2A, 2B, 2a and 2b can be improved.
However, in the conventional encoder device 1 having the structure described above, when a given slit 4 of the main scale 5 passes a position opposite a central portion of the lens 3a of the light-emitting element 3, the light emitted from the light-emitting element 3 via the lens 3a is not in the form of parallel beams of light but is dispersed at predetermined angles and, at the same time, diffracted by the edges of the slits 4, and thus light leaks from the slits 4. As a result, the light-receiving elements 2A and 2b, which are positioned near the central portion of the lens 3a, are affected by the above-described leaked light and the detectional accuracy of the light-receiving elements 2A and 2b is degraded.
Moreover, although it is desirable to make the encoder device slimmer, the effect of the above-described leaked light only increases as the light-receiving elements 2A, 2B, 2a and 2b are positioned closer to the lens 3a in an effort to make the encoder device slimmer.
It should be noted that although in FIG. 1 the leaked light appears to penetrate the main scale 5, in actuality the leaked light is cut off by the main scale 5 (the slanted line sections shown in FIG. 1) once a given slit 4 has passed the position opposite the central portion of the lens 3a, and hence does not strike the light-receiving elements 2A and 2b. 
Further, the volume of light is particularly heavy around a central axis and surrounding area of the lens 3a, and as a result the effect of leaked light tends to be more pronounced thereabout. Thus light-receiving elements 2A and 2b are particularly susceptible to the effects of leaked light because they are positioned closer to the central portion of the lens 3a than light-receiving elements 2B and 2a. 
As a result, the accuracy and reliability of the phase A signal and the phase b signal output from the light-receiving elements 2A and 2b declines.
Accordingly, it is an object of the present invention to provide an improved and useful encoder device in which the above-mentioned disadvantages are eliminated.
The above-described object of the present invention is achieved by an encoder device comprising:
a main scale with slit-like openings at regular intervals;
light-emitting means for emitting and directing light toward the main scale;
light-receiving means including four light receiving members for receiving light emitted from the light-emitting means via the slit-like openings in the main scale; and
means for obtaining information concerning displacement of the main scale by using output signals output from the light-receiving means,
the first light-receiving member and the second light-receiving member disposed with respect to the slit-like openings of the main scale so as to have substantially the same phase,
the third light-receiving member and the fourth light-receiving member disposed with respect to the slit-like openings of the main scale so that a first differential output signal, obtained by differentially amplifying an output signal output from the first light-receiving member and an output signal output from the third light-receiving member, and a second differential output signal, obtained by differentially amplifying an output signal output from the second light-receiving member and an output signal output from the fourth light-receiving member, have the same period, and further, the first differential output signal and the second differential output signal have a predetermined phase difference.
According to the invention described above, the effect of light leaking from the openings in the main scale in the area of the central axis of the light-emitting means can be eliminated and the distance separating the lens and the light-receiving members can be reduced. As a result, the encoder device can be made slimmer.
Additionally, the above-described object of the present invention is also achieved by the encoder device as described above, wherein the output signal output from the third light-receiving member has a first phase difference with respect to the output signal output from the first light-receiving member, and the output signal output from the fourth light-receiving member has a second phase difference with respect to the output signal output from the second light-receiving member.
According to the invention described above, the first, second, third and fourth light-receiving members can be positioned so that the first differential output signal and the second differential output signal have a predetermined phase difference.
Additionally, the above-described object of the present invention is also achieved by the encoder device described above, wherein the light-emitting means has a lens for emitting light in substantially parallel beams, a central axis of the lens being positioned along a line midway between the first light-receiving member and the second light-receiving member.
According to the invention described above, the first light-receiving member and the second light-receiving member can be positioned so as to have substantially the same phase with respect to the openings of the main scale.
Additionally, the above-described object of the present invention is also achieved by the encoder device described above, wherein the first phase difference is approximately 135 degrees and the second phase difference is approximately 45 degrees.
According to the invention described above, a first differential output signal and a second differential output signal having a phase difference of 90 degrees can be output.
Additionally, the above-described object of the present invention is also achieved by the encoder device described above, wherein a solid shield portion of the main scale has a width identical to a width of the slit-like openings of the main scale.
According to the invention described above, the light emitted from the light-emitting means can be received at the light-receiving members in such a way as to reflect accurately the relative displacement between the main scale and each of the light-receiving members.
Additionally, the above-described object of the present invention is also achieved by the encoder device described above, wherein the four light-receiving members are disposed substantially in an arc so as to surround the central axis of the lens of the light-emitting means.
According to the invention described above, each of the light-receiving members can be positioned near the spot of light emitted from the light-emitting means and an appropriate signal level output from the light-receiving members can be maintained.
Additionally, the above-described object of the present invention is also achieved by the encoder device described above, wherein the predetermined phase difference is 90 degrees.
According to the invention described above, the phase difference of both signals is easy to detect and positional detection accuracy can be maintained.
Other objects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings.